1. Introduction
The growing global demand for energy and the high use of fossil fuels are a matter of distress in both the long and short term, as these energy sources are not renewable [
1,
2]. There may be a shortage of these non-renewable energy sources in the future, which could result in economic and political conflicts between energy-scarce nations. Therefore, there is an increasing urgency to research for renewable energy sources that can meet humanity’s energy needs in the long term [
3,
4]. Hungary is poor in fossil fuels, but half of its area is under arable cultivation, and its agro-ecological characteristics also favor biomass production. For this reason, energy produced as biomass as an alternative energy source may be the main perspective in the future [
5]. Areas that do not allow the successful cultivation of other crops can be used to for energy crops. At the same time, they meet the growing conditions for some woody or herbaceous energy plants. The sorghum plant may be a perfect candidate for the production of low-cost biofuels in the future, as its abiotic stress tolerance, diverse genetics, and reliable seed production all contribute to this property [
6].
During changes in climatic conditions, drought periods and uneven rainfall distribution become more frequent. It has been described several studies that sorghum has excellent drought tolerance, with dry-land regions growing more than maize. In drought conditions, sorghum grain absorbs nutrients more efficiently than maize. However, the sorghum crop grown under non-irrigated conditions does not exceed the irrigated crop [
7,
8,
9]. Declining freshwater supplies and pollution are global problems [
10]. According to a study by Mekonnen and Hoekstra [
11], approximately four billion people live in water scarcity worldwide, and an estimated five hundred million people live in areas with grave water crisis. For this reason, one of the most significant resources today is water. Nowadays, the biggest challenge is to provide irrigation water for agriculture in the context of the increasingly frequent drought phenomenon. As a consequence of climate change periodically and regionally, there may be a phenomenon in which the surface freshwater supply is insufficient to meet irrigation water demand [
12]. The need for irrigation water can be solved by making more optimal use of the available irrigation water. However, in some situations, it may be necessary to use municipal wastewater or agricultural effluent water [
13]. Municipal water sources contain lower concentrations of potential pollutants compared to industrial wastewater [
14]. During the use of reused water in agriculture, environmental changes that may have a positive or negative property should be monitored [
15]. Another source is the irrigation utilization of the effluent of intensive aquaculture systems. Moreover, the effluent is usually rich in organic matter; therefore, the fertilizer doses applied to the production area can also be reduced [
16]. At the same time, nutrient accumulation caused by large amounts of organic and inorganic metabolites and residual fish feed should be taken into account when placing effluents in natural recipients [
17].
The importance of growing sorghum is increased by the fact that it does not require as intensive plant protection and nutrient replenishment as maize [
18]. It is less sensitive to the quality of the area and can be grown successfully in places where other crops make little or no profit in an average year. The uses of sorghum are diverse. Sorghum also plays a significant role in human consumption; in terms of production area, it ranks fifth after maize, rice, wheat and barley among the cereal crops [
19]. Sorghum is a plant of physiological type C
4 with high productivity and good drought tolerance [
20,
21]. Species have good drought tolerance due to their original habitat conditions; indeed, sorghum’s gene center is the steppe and savannah region of Africa [
22]. It has a water demand of 500–580 mm/year and a transpiration coefficient of 150–250 l/kg dry matter [
8]. It can be used as a multi-purpose energy crop in both human food and feed production, although it can also be grown for energy purposes [
23,
24].
The aim of our study was to determine the growth rate of the sorghum varieties that were irrigated with effluent water from an intensive fish farm with a higher Na content, and to define the biomass production. Our further objective was to determine the concentration of N, P, K and Na elements which were accumulated in plant parts and its effect on the macronutrient content in the soil.
4. Discussion
The irrigation experiment applied to the grain sorghum varieties took place between 2016 and 2020. With population growth and swift urbanization, the agricultural sector is under increasing pressure as freshwater supplies for crop production declining in all parts of the world [
25]. The use of wastewater and effluents of industrial or agricultural origin is an essential element in the protection of our water resources. According to Qi et al. [
26], effluents from aquaculture systems have a rich organic matter content which can be advantageously used in crop production. At the same time, it increases soil fertility, improves cultivation success and reduces fertilizer costs [
27]. Irrigation of the higher salinity effluent with us gave similar results as Guimarães et al. [
28] described in their research that the cultivation of the sorghum forage could be solved by irrigation with saline effluent water.
The high Na
+ and HCO
3− concentration in irrigation water is known to be responsible for soil salinization. In sodic soils, ionic exchange between Na
+ and H
+ causes the dissociation of water in soil solution, leading to increasing concentrations of NaOH in the soil solution and the soil pH may increase to values above 10.5 [
29,
30]. The negative relationship between basic respiration and pH in salt-affected soils [
31] could be another reason of the alkalization of the soil irrigated with effluent water (
Table 4). In case of total carbonate, total organic carbon and N values, there were no significant differences due to the treatments (
Table 4).
According to our results in the non-irrigated treatment, the highest EC value was measured (in surface soil layer). Strong correlation was found between EC and P and K content of the soil (Pearson correlation coefficients 0.824 and 0.823, respectively, sig. < 0.01) in the surface layer, but there was no correlation between them in the subsoil layer. We assume the EC differences occur because of the more available nutrients (P, K) in the soil at 0–30 cm depth.
The impact of irrigation and water quality on the available phosphorus content of the soil was proved in the surface soil layer where the lowest mean P content were calculated in E30 and E45, despite the P content of the effluent and the river water (
Table 1 and
Table 4). We assume the disintegration of soil aggregates was due to soil salinization was the significant role of the released colloid-sized clay particles in P fixation; however, further studies are needed to prove our assumption. According to Arienzo et al. [
32], potassium availability is strongly affected by the pH of the wastewater, as well as by the pH of the receiving soil. Normally, potassium availability is sustained for most plants in neutral or slightly acidic soils. In this study, the pH was significantly lower in the soil irrigated with Körös River water, which may have caused higher K content in the control treatment.
One of the acidic extractants, the ammonium lactate (AL, pH = 3.7) solution, introduced by Egner et al. [
33], is commonly used in Europe. When the soil is treated with AL extraction solution, the soluble substance enters the solution partly through dissolution and partly through ion exchange, and AL extraction solution could decompose the carbonates also. The higher sodium concentration of the soils irrigated with effluent (
Table 4) indicates the start of the sodication process.
The increase in the nitrogen content of sorghum plants is directly proportional to the higher crude protein content [
34], which may mean a more nutritious feed for the animals. Although, the lower nitrogen content affects the physiology of the plant processes in which the macronutrient content of the grain changes, in particular the uptake of Ca, Mg and S [
34]. For all three cultivars, significantly higher nitrogen values were measured in the grain yield in the first two growing years. In certain years of the experiment, it was found for each variety that the higher N content of the effluent irrigation water could be well utilized in grain yields.
The P demand of plants is high during the development of vegetative organs, but it is also significant during crop production. The seeds are the phosphorus-containing plant organs [
35]. Nitrogen and phosphorus are antagonists of each other in terms of their physiological effects, where N stimulates the growth of vegetative organs, while phosphorus stimulates the appearance of generative organs and crop ripening [
36]. Regarding phosphorus, there was no significant difference between the varieties and the irrigation treatments. On the other hand, the ‘Alföldi 1’ and ‘GK Emese’ varieties were able to make slightly better use of the higher P content of effluent irrigation water, especially in the last two years.
Potassium is an essential element for growth and one of the most frequently occurring cations in plant organs. Unlike other elements such as nitrogen, phosphorus, magnesium, calcium and sulfur, potassium is not incorporated into organic matter. Over time, the K content of older organs showed a decreasing trend. [
37]. In the experiment, sorghum plants had high K levels of grain yield between 3500 and 5000 mg/kg d.m. There was no significant difference between the varieties. The Na
+/K
+ ratio is considered to be the basis of the salt tolerance of plants, which increases in direct proportion to the increase in salinity [
38]. According to Ahmad et al. [
39] and Iqbal et al. [
40] studies, effluent irrigation water with higher salinity did not reduce the accumulation of K
+ in plant organs.
High salinity in plants cause hyperionic and hyperosmotic stress effects, as well as limited growth. Sodium is not essential even for extreme salt-tolerant plants, requiring only small amounts of C
4 and CAM plants [
41,
42]. Due to this, in addition to salt stress, sorghum is able to maintain its photosynthetic activity and dry matter production [
43]. The sodium content of the grain crop was the lowest in the first year of cultivation. There was a difference in the accumulation of the cultivars, where a higher Na level was detected in the case of ‘Farmsugro 180’, while the lowest sodium content was measured in ‘GK Emese’. The level of Na in the grain yield of sorghum also shows an upward trend between the years, but it occurred to different degrees for the three sorghum cultivars. However, this value has not yet been shown to be toxic to the dose. In a vegetation period—in proportion to the amount of annual irrigation—41–66 g/m
2 Na was applied in the case of 30 mm effluent irrigation and 49–99 g/m
2 Na in the case of 45 mm effluent.
Sixto et al. [
44] have shown that a decrease in vegetative growth parameters can be observed in plants as a function of increasing salinity. In plants exposed to salt stress, a decrease in shoot, stem and root development, fresh and dry stem and root mass, leaf area and number of leaf, and relative chlorophyll amount and yield were observed [
45,
46,
47,
48]. For all three varieties, the average SPAD value of the leaves was lower in the last two growing years. There is a linear relationship between the nitrogen content of the crop and the chlorophyll value, where a positive correlation (
r = 0.737, Pearson correlation) was observed during the study.
In the case of plant height, it can be stated that the highest plants (149–236 cm) were detected in the first year of cultivation, which can be explained mainly by the maximum amount of total water (precipitation + irrigation). Subsequently, a decrease was observed for all three cultivars (133–181 cm), depending on the total annual water volumes, as plant height is primarily affected by precipitation and temperature. In the experiment, the ‘Farmsugro 180’ fell short of its average height of 180–220 cm except for the first year. However, the measured height data of ‘Alföldi 1’ (140–16 cm), and mainly the ‘GK Emese’ (130–150 cm), corresponded to their characteristic height, which means that they were well adapted to the experimental stress conditions. This trend is also observed in the weight of grain yield. In addition, several studies have reported that higher salinity in irrigation water reduces plant mass, crop, and biomass product [
49,
50].
Hussein et al. [
51] showed that higher Na concentrations of irrigation water had a negative effect on the growth profile of sorghum. In the 2017 growing year, the amount of irrigation water presented a positive correlation (
r = 0.026, Pearson correlation) for both green mass and grain yield. In the case of both green mass and grain yield, it was observed that lower biomass values were detected in the last year of cultivation. The sorghum is a moderately salt tolerant plant [
52], and no yield reduction is expected at irrigation water with EC of 4.5 dS/m and soil salinity up to 6.8 dS/m. According to our soil EC values (
Table 4), it is not proven that salinity could cause the decrease; however, a detailed analyses of soil exchangeable sodium percentage would be justified to further investigate the effluent water impact on these sorghum cultivars. Nevertheless, a decrease occurred in all treatments, and hence cannot be linked to water quality with absolute certainty. For example, the sensitivity of ‘Farmsugro 180’ should be emphasized, during which the value of grain yield in the case of samples irrigated with the water of the Körös River in the last year was only between 57–67 g/plant.